Optical fiber with large effective area and low bending loss

ABSTRACT

An optical fiber with large effective area, low bending loss and low attenuation. The optical fiber includes a core, an inner cladding region, and an outer cladding region. The core region includes a spatially uniform updopant to minimize low Rayleigh scattering and a relative refractive index and radius configured to provide large effective area. The inner cladding region features a large trench volume to minimize bending loss. The core may be doped with Cl and the inner cladding region may be doped with F.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 62/186,768 filed on Jun. 30, 2015and Provisional Application Ser. No. 62/316,767 filed on Apr. 1, 2016the content of which is relied upon and incorporated herein by referencein its entirety.

FIELD

The present description relates to optical fibers having large effectivearea and low attenuation. More particularly, the present descriptionrelates to optical fibers exhibiting high core doping concentration,high core doping uniformity and low core stress.

BACKGROUND

Low attenuation optical fibers are desirable for many applications,including signal transmission over long distances. To achievewaveguiding, the optical fiber requires a high index core and a lowindex cladding with an adequate core-cladding index differential. Mostoptical fibers incorporate germania (GeO₂) as an updopant(index-increasing dopant) in silica for the core and use undoped silicafor the cladding. Fluctuations in the concentration of germania,however, lead to high attenuation due to Rayleigh scattering and limitthe use of germania-doped fibers in low loss applications.

An alternative approach is to design the fiber with an undoped silicacore and to include a downdopant in the silica cladding to achieve thecore-cladding index differential needed for effective waveguiding. Themost common downdopant for silica is fluorine. This approach suffersfrom two drawbacks. First, undoped silica has a high melt viscosity andproduces a core having a high fictive temperature upon cooling of themelt at practical rates. The high fictive temperature is indicative ofan unrelaxed structural state of the core silica glass and increasesfiber attenuation through Rayleigh scattering. Second, doping a silicacladding with fluorine lowers the melt viscosity of the cladding. Inorder to achieve the core-cladding differential needed for effectivewaveguiding with an undoped silica core, however, the fluorine dopingconcentration in the cladding needs to be high. Although high fluorineconcentrations can be achieved, incorporation of fluorine as a dopant atthe necessary concentration leads to a significant reduction in the meltviscosity of the cladding. As a result, a large viscosity mismatchdevelops between the core and cladding regions during draw. The largeviscosity mismatch leads to significant stresses in the core during cooldown and is responsible for a stress-optic effect that lowers the indexof the core, thus compromising the waveguiding characteristics of thefiber by reducing confinement. Alleviation of core stresses and stressoptic effects requires drawing of fibers at speeds slow enough to relaxstresses and equilibrate the structure of the fiber. The necessaryspeeds, however, are too slow for practical manufacturing.

There remains a need for fibers having low attenuation that can bemanufactured at high speeds.

SUMMARY

Disclosed herein are optical waveguide fibers. The fibers feature lowattenuation, large effective area, and low bending losses.

The optical fibers include a core and a cladding. The core is updopedsilica glass and the cladding includes an inner cladding region and anouter cladding region. The inner cladding region is downdoped silicaglass or undoped silica glass. The outer cladding region is silica glassand may be undoped, updoped, or downdoped.

In one embodiment, the core is silica glass doped with chlorine, theinner cladding region is silica glass doped with fluorine and the outercladding region is undoped silica glass. In another embodiment, the coreis silica glass doped with chlorine, the inner cladding region is silicaglass doped with fluorine and the outer cladding region is silica glassdoped with chlorine. In still another embodiment, the core is silicaglass doped with chlorine, the inner cladding region is silica glassdoped with fluorine and the outer cladding region is silica glass dopedwith a lower concentration of fluorine than the inner cladding region.In yet another embodiment, core is silica glass doped with chlorine, theinner cladding region is undoped silica glass and the outer claddingregion is silica glass doped with chlorine.

In one embodiment, the chlorine-doped silica glass core is free of Ge.In another embodiment, the chlorine-doped silica glass core is free ofK. In still another embodiment, the chlorine-doped silica glass core isfree of Ge and K.

The doping concentration of Cl in the core may be in the range from 1.1wt %-3.0 wt %, or in the range from 1.5 wt %-3 wt %, or in the rangefrom 1.5 wt %-2.75 wt %, or in the range from 1.5 wt %-2.5 wt %, or inthe range from 1.75 wt %-3.0 wt %, or in the range from 1.75 wt %-2.75wt %, or in the range from 1.75 wt %-2.5 wt %, or greater than 1.5 wt %,or greater than 1.75 wt %, or greater than 2.0 wt %, or greater than2.25 wt %. The doping concentration of F in the inner cladding regionmay be in the range from 0.10 wt %-0.50 wt %, or in the range from 0.15wt %-0.45 wt %, or in the range from 0.20 wt %-0.40 wt %, or at least0.10 wt %, or at least 0.15 wt %, or at least 0.20 wt %. In preferredembodiments, the concentration of chlorine in the core is greater than1.5 weight %.

The doping concentration of Cl in the silica glass core may be in therange from 2.0 wt %-3.0 wt % and the inner cladding region may beundoped silica glass, or the doping concentration of Cl in the silicaglass core may be in the range from 2.2 wt %-2.8 wt % and the innercladding region may be undoped silica glass, or the doping concentrationof Cl in the silica glass core may be in the range from 2.3 wt %-2.7 wt% and the inner cladding region may be undoped silica glass.

In one embodiment, the outer cladding region is silica glass doped withCl and the Cl concentration is in the range from 0.10 wt %-0.60 wt %, orin the range from 0.20 wt %-0.50 wt %. In another embodiment, the outercladding region is silica glass doped with F and the F concentration isin the range from 0.05 wt %-0.30 wt %, or in the range from 0.10 wt%-0.25 wt %.

The relative refractive index Δ₁ of the core may be in the range from0.08% to 0.30%, or in the range from 0.10% to 0.25%, or in the rangefrom 0.12% to 0.20%, or in the range from 0.14% to 0.18%. The radius r₁of the core may be in the range from 5.0 μm to 9.0 μm, or in the rangefrom 6.0 μm to 10.0 μm, or in the range from 6.0 μm to 9.0 μm, or in therange from 6.0 μm to 8.0 μm, or in the range from 6.5 μm to 7.5 μm, orin the range from 7.0 μm to 10.0 μm.

The relative refractive index Δ₂ of the inner cladding region may be inthe range from 0% to −0.25%, or in the range from −0.05% to −0.20%, orin the range from −0.10% to −0.20%. The radius r₂ of the inner claddingregion may be in the range from 15 μm to 40 μm, or in the range from 15μm to 38 μm, or in the range from 20 μm to 38 μm, or in the range from20 μm to 35 μm, or in the range from 20 μm to 30 μm, or in the rangefrom 22 μm to 38 μm, or in the range from 22 μm to 35 μm, or in therange from 24 μm to 38 μm, or in the range from 24 μm to 35 μm.

The relative refractive index Δ₃ of the outer cladding region may be inthe range from −0.20% to 0.10%, or in the range from −0.15% to 0.10%, orin the range from −0.10% to 0.05%, or in the range from −0.05% to 0.05%.The radius r₃ of the outer cladding region may be at least 55 μm, or atleast 60 μm, or in the range from 55 μm to 70 μm, or in the range from60 μm to 65 μm, or about 62.5 μm.

The relative refractive index difference Δ₁−Δ₂ may be at least 0.15%, orat least 0.20%, or at least 0.25%, or at least 0.30%. The relativerefractive index difference Δ₃−Δ₂ may be at least 0.05%, or at least0.06%, or at least 0.08%, or at least 0.10% or at least 0.12%, or atleast 0.15%, or at least 0.20%.

The mode field diameter of the fiber at a wavelength of 1550 nm may beat least 10.0 μm, or at least 11.0 μm, or at least 11.5 μm, or at least12.0 μm, at least 12.5 μm, or at least 13.0 μm, or at least 13.5 μm, orat least 14.0 μm, or in the range from 10.0 μm to 15.0 μm, or in therange from 11.0 μm to 14.0 μm.

The effective area of the present fibers at a wavelength of 1550 nm maybe at least 100 μm², or at least 110 μm², or at least 120 μm², or atleast 130 μm², or at least 140 μm², or at least 150 μm², or in the rangefrom 100 μm² to 180 μm², or in the range from 110 μm² to 165 μm², or inthe range from 120 μm² to 155 μm².

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 20 mm may be lessthan 4.0 dB/turn, or less than 3.5 dB/turn, or less than 3.0 dB/turn, orless than 2.5 dB/turn, or less than 2.0 dB/turn, or less than 1.5dB/turn, or less than 1.0 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 30 mm may be lessthan 1.5 dB/turn, or less than 1.0 dB/turn, or less than 0.8 dB/turn, orless than 0.6 dB/turn, or less than 0.4 dB/turn, or less than 0.3dB/turn, or less than 0.2 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 40 mm may be lessthan 0.6 dB/turn, or less than 0.4 dB/turn, or less than 0.2 dB/turn, orless than 0.1 dB/turn, or less than 0.05 dB/turn, or less than 0.025dB/turn, or less than 0.01 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 50 mm may be lessthan 0.25 dB/turn, or less than 0.10 dB/turn, or less than 0.05 dB/turn,or less than 0.02 dB/turn, or less than 0.01 dB/turn, or less than 0.005dB/turn, or less than 0.002 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 60 mm may be lessthan 5 dB/100 turns, or less than 3 dB/100 turns, or less than 2 dB/100turns, or less than 1 dB/100 turns, or less than 0.5 dB/100 turns, orless than 0.2 dB/100 turns, or less than 0.1 dB/100 turns.

The attenuation of the present fibers at 1550 nm may be less than 0.18dB/km, or less than 0.175 dB/km, or less than 0.17 dB/km, or less than0.165 dB/km, or less than 0.16 dB/km.

The cable cutoff wavelength of the present fibers may be less than 1550nm, or less than 1500 nm, or less than 1450 nm, or less than 1400 nm.

The present disclosure extends to:

-   An optical fiber comprising:

a core region comprising Cl-doped silica glass having a chlorineconcentration greater than 1.5 wt %, said core region having an outerradius r₁ in the range from 6.0 microns to 10.0 microns and a relativerefractive index Δ₁;

an inner cladding region surrounding said core region, said innercladding region having an outer radius r₂ in the range from 22 micronsto 38 microns and a relative refractive index Δ₂; and

an outer cladding region surrounding said inner cladding region, saidouter cladding region having a relative refractive index Δ₃, saidrelative refractive index Δ₃ exceeding said relative refractive index Δ₂by at least 0.06%;

wherein said optical fiber has a cable cutoff of less than 1550 nm, aneffective area at 1550 nm of at least 100 micron, and a bending loss at1550 nm, determined from a mandrel wrap test using a mandrel with adiameter of 20 mm, of less than 3.5 dB/turn.

The present disclosure extends to:

-   A method of making an optical fiber comprising:

forming soot from an organosilicon compound; and

doping said soot with Cl, and

sintering said doped soot, said sintered doped soot having a Clconcentration of at least 0.8 wt %.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent description, and together with the specification serve toexplain principles and operation of methods, products, and compositionsembraced by the present description. Features shown in the drawing areillustrative of selected embodiments of the present description and arenot necessarily depicted in proper scale.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the written description,it is believed that the specification will be better understood from thefollowing written description when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a schematic depiction in cross-section of a fiber having acore, inner cladding region, outer cladding region, a primary coatingand a secondary coating.

FIG. 2 depicts a schematic refractive index profile of the glass portionof an optical fiber.

FIG. 3 is a schematic depiction of soot preform deposition via an OVDprocess.

FIG. 4 depicts an apparatus for doping and consolidating a soot preform.

FIGS. 5-18 depict illustrative relative refractive index profiles.

The embodiments set forth in the drawings are illustrative in nature andnot intended to be limiting of the scope of the detailed description orclaims. Whenever possible, the same reference numeral will be usedthroughout the drawings to refer to the same or like feature.

DETAILED DESCRIPTION

Reference will now be made in detail to illustrative embodiments of thepresent description.

The present description provides optical fibers that exhibit lowattenuation, low bending loss, and high effective area. The opticalfibers include a core and a cladding that surrounds the core. Thecladding includes an inner cladding region and an outer cladding region.

An explanation of selected terms as used herein is now provided:

“Radial position” or the radial coordinate “r” refers to radial positionrelative to the centerline (r=0) of the fiber. The length dimension“micron” may referred to herein as micron (or microns) or μm. Arealdimensions based on microns may be referred to herein as micron² or μm².

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and fiber radius. For relativerefractive index profiles depicted herein as having step boundariesbetween adjacent core and/or cladding regions, normal variations inprocessing conditions may preclude obtaining sharp step boundaries atthe interface of adjacent regions. It is to be understood that althoughboundaries of refractive index profiles may be depicted herein as stepchanges in refractive index, the boundaries in practice may be roundedor otherwise deviate from perfect step function characteristics. It isfurther understood that the value of the relative refractive index mayvary with radial position within the core region and/or any of thecladding regions. When relative refractive index varies with radialposition in a particular region of the fiber (core region and/or any ofthe cladding regions), it may be expressed in terms of its actual orapproximate functional dependence or in terms of an average valueapplicable to the region. Unless otherwise specified, if the relativerefractive index of a region (core region and/or any of the claddingregions) is expressed as a single value, it is understood that therelative refractive index in the region is constant, or approximatelyconstant, and corresponds to the single value or that the single valuerepresents an average value of a non-constant relative refractive indexdependence with radial position in the region. Whether by design or aconsequence of normal manufacturing variability, the dependence ofrelative refractive index on radial position may be sloped, curved, orotherwise non-constant.

The “relative refractive index” or “relative refractive index percent”of an optical fiber is defined as:

${\Delta \mspace{14mu} \%} = {100\frac{{n^{2}(r)} - n_{c}^{2}}{2{n^{2}(r)}}}$

where n(r) is the refractive index of the fiber at the radial distance rfrom the fiber's centerline, unless otherwise specified, and n_(c) is1.444, which the refractive index of undoped silica glass at awavelength of 1550 nm. As used herein, the relative refractive index isrepresented by Δ (or “delta”) or Δ % (or “delta %) and its values aregiven in units of “%”, unless otherwise specified. Relative refractiveindex may also be expressed as Δ(r) or Δ(r)%.

The average relative refractive index of a region of the fiber isdetermined from:

${\Delta \mspace{14mu} \%} = \frac{\int_{r_{inner}}^{r_{outer}}{{\Delta (r)}{r}}}{\left( {r_{outer} - r_{inner}} \right)}$

where r_(inner) is the inner radius of the region, r_(outer) is theouter radius of the region, and Δ(r) is the relative refractive index ofthe region.

The term “α-profile” (also referred to as an “alpha profile”) refers toa relative refractive index profile Δ(r) that has the followingfunctional form:

${\Delta (r)} = {{\Delta \left( r_{0} \right)}\left\lbrack {1 - \left\lbrack \frac{{r - r_{0}}}{\left( {r_{1} - r_{0}} \right)} \right\rbrack^{\alpha}} \right\rbrack}$

where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) is zero, and r is in the range r_(i)≦r≦r_(f), where r_(i) isthe initial point of the α-profile, r_(f) is the final point of theα-profile, and α is a real number. In some embodiments, examples shownherein can have a core alpha of 1≦α≦100. In some embodiments, examplesshown herein can have a core alpha of 1≦α≦10. In some embodiments,examples shown herein can have a core alpha of 10≦α≦100. In someembodiments, examples shown herein can have a core alpha of 10≦α≦30.

“Effective area” of an optical fiber is defined as:

$A_{eff} = \frac{2\; {\pi \left\lbrack {\int_{0}^{\infty}{\left( {f(r)} \right)^{2}r{r}}} \right\rbrack}^{2}}{\int_{0}^{\infty}{\left( {f(r)} \right)^{4}r{r}}}$

where f(r) is the transverse component of the electric field of theguided optical signal and r is radial position in the fiber. “Effectivearea” or “A_(eff)” depends on the wavelength of the optical signal andis understood herein to refer to a wavelength of 1550 nm. Specificindication of the wavelength will be made when referring to “Effectivearea” or “A_(eff)” herein.

The “mode field diameter” or “MFD” of an optical fiber is defined as:

M F D = 2w$w^{2} = {2\frac{\int_{0}^{\infty}{\left( {f(r)} \right)^{2}r{r}}}{\int_{0}^{\infty}{\left( \frac{{f(r)}}{r} \right)^{2}r{r}}}}$

where f(r) is the transverse component of the electric fielddistribution of the guided optical signal and r is radial position inthe fiber. “Mode field diameter” or “MFD” depends on the wavelength ofthe optical signal and is understood herein to refer to a wavelength of1550 nm.

“Trench volume” is defined as:

V _(Trench)=|2∫_(r) _(Trench,inner) ^(r) ^(Trench,outer)Δ_(Trench)(r)rdr|

where r_(Trench,inner) is the inner radius of the trench region of therefractive index profile, r_(Trench,outer) is the outer radius of thetrench region of the refractive index profile, Δ_(Trench)(r) is therelative refractive index of the trench region of the refractive indexprofile, and r is radial position in the fiber. Trench volume is inabsolute value and a positive quantity and will be expressed herein inunits of % Δmicron², % Δ-micron², % Δ-μm², or % Δμm², whereby theseunits can be used interchangeably herein.

“Chromatic dispersion”, herein referred to as “dispersion” unlessotherwise noted, of an optical fiber is the sum of the materialdispersion, the waveguide dispersion, and the intermodal dispersion. Inthe case of single mode waveguide fibers, the inter-modal dispersion iszero. Dispersion values in a two-mode regime assume intermodaldispersion is zero. The zero dispersion wavelength (λ₀) is thewavelength at which the dispersion has a value of zero. Dispersion slopeis the rate of change of dispersion with respect to wavelength.Dispersion and dispersion slope are reported herein at a wavelength of1550 nm and are expressed in units of nm/ps/km.

The cutoff wavelength of an optical fiber is the minimum wavelength atwhich the optical fiber will support only one propagating mode. Forwavelengths below the cutoff wavelength, multimode transmission mayoccur and an additional source of dispersion may arise to limit thefiber's information carrying capacity. Cutoff wavelength will bereported herein as a fiber cutoff wavelength or a cable cutoffwavelength. The fiber cutoff wavelength is based on a 2-meter fiberlength and the cable cutoff wavelength is based on a 22-meter cabledfiber length. The 22-meter cable cutoff wavelength is typically lessthan the 2-meter cutoff wavelength due to higher levels of bending andmechanical pressure in the cable environment.

The bend resistance of an optical fiber may be gauged by bend-inducedattenuation under prescribed test conditions. In the presentdescription, bend losses were determined by a mandrel wrap test. In themandrel wrap test, the fiber is wrapped around a mandrel having aspecified diameter and the attenuation of the fiber in the wrappedconfiguration at 1550 nm is determined. The bend loss is reported as theincrease in attenuation of the fiber in the wrapped configurationrelative to the attenuation of the fiber in an unwrapped (straight)configuration. Bend loss is reported herein in units of dB/turn, whereone turn corresponds to a single winding of the fiber about thecircumference of the mandrel. Bend losses for mandrel diameters of 20mm, 30 mm, 40 mm, 50 mm, and 60 mm were determined.

In the wire mesh covered drum test for measuring micro-bending, a 400 mmdiameter aluminum drum is wrapped with wire mesh. The mesh is wrappedtightly without stretching. The wire mesh should be intact withoutholes, dips, or damage. The wire mesh material used in the measurementsherein was made from corrosion-resistant type 304 stainless steel wovenwire cloth and had the following characteristics: mesh per linear inch:165.times.165, wire diameter: 0.0019″, width opening: 0.0041″, and openarea %: 44.0. A prescribed length (750 m) of waveguide fiber is wound at1 m/s on the wire mesh drum at 0.050 cm take-up pitch while applying 80(+/−1) grams of tension. The ends of the prescribed length of fiber aretaped to maintain tension and there are no fiber crossovers. Theattenuation of the optical fiber is measured at a selected wavelength(typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or1625 nm). A reference attenuation is measured for the optical fiberwound on a smooth drum (i.e. a drum without a wire mesh). The increasein fiber attenuation (in dB/km) in the measurement performed on the drumwith the wire mesh relative to the measurement performed on the smoothdrum is reported as the wire mesh covered drum attenuation of the fiberat the selected wavelength.

The present fibers include a core and a cladding surrounding the core.The fibers may also include a primary coating surrounding the claddingregion, and a secondary coating surrounding the primary coating. Thecladding may be directly adjacent the core. The primary coating may bedirectly adjacent the cladding. The secondary coating may be directlyadjacent the primary coating. The cladding region may include an innercladding region and an outer cladding region. The outer cladding regionmay be directly adjacent the inner cladding region. The inner claddingregion may be directly adjacent the core. The primary coating may bedirectly adjacent the outer cladding region. As used herein, “directlyadjacent” means in direct physical contact with, where direct physicalcontact refers to a touching relationship. In alternative embodiments,intervening layers or regions may be present between the core andcladding, or between the cladding and primary coating, or between theprimary coating and secondary coating, or between the inner claddingregion and core, or between the outer cladding region and the innercladding regions, or between the primary coating and the outer claddingregion. Elements within a fiber that are separated by one or moreintervening regions or layers are referred to herein as being “adjacent”and are not in direct physical contact with each other.

The refractive index profile of the core region may be designed tominimize attenuation losses while maintaining a large effective area forthe fiber. The primary and secondary coatings may be selected to protectthe mechanical integrity of the core and cladding and to minimize theeffects of external mechanical disturbances on the characteristics ofthe optical signal guided in the fiber. The primary and secondarycoatings may insure that losses due to bending and other perturbingforces are minimized. The index profile of the cladding region may alsobe designed to contribute to a reduction in bending losses.

Whenever used herein, radius r₁ and relative refractive index Δ₁(r)refer to the core, radius r₂ and relative refractive index Δ₂(r) referto the inner cladding region, and radius r₃ and relative refractiveindex Δ₃(r) refer to the outer cladding region. It is understood thatthe core forms the central portion of the fiber and is substantiallycylindrical in shape. It is further understood that the surroundinginner cladding region and surrounding outer cladding region aresubstantially annular in shape. Annular regions may be characterized interms of an inner radius and an outer radius. Radial positions r₁, r₂,and r₃ refer herein to the outermost radii of the core, inner claddingregion, and outer cladding region, respectively. When two regions aredirectly adjacent to each other, the outer radius of the inner of thetwo regions coincides with the inner radius of the outer of the tworegions. In one embodiment, for example, the fiber includes an innercladding region surrounded by and directly adjacent to an outer claddingregion. In such an embodiment, the radius r₂ corresponds to the outerradius of the inner cladding region and the inner radius of the outercladding region.

In certain embodiments, the relative refractive index profile mayinclude an offset region between the core and inner cladding region. Theradius r₄ and relative refractive index Δ₄(r) refer to the offsetregion. The radius r₄ refers to the outermost radius of the offsetregion. When an offset region is present in the relative refractiveindex profile, r₁ corresponds to the outer radius of the core and theinner radius of the offset region, while r₄ corresponds to the outerradius of the offset region and the inner radius of the inner claddingregion.

As will be described further hereinbelow, the relative refractiveindices of the core, inner cladding region, and outer cladding regionmay differ. Each of the regions may be formed from silica glass or asilica-based glass. A silica-based glass is silica glass doped ormodified with one or more elements. Variations in refractive index maybe accomplished by incorporating updopants or downdopants at levelsknown to provide a targeted refractive index or refractive index profileusing techniques known to those of skill in the art. Updopants aredopants that increase the refractive index of the glass relative to theundoped glass composition. Downdopants are dopants that decrease therefractive index of the glass relative to the undoped glass composition.In one embodiment, the undoped glass is pure silica glass. When theundoped glass is pure silica glass, updopants include Cl, Br, Ge, Al, P,Ti, Zr, Nb, and Ta, and downdopants include F and B. Regions of constantrefractive index may be formed by not doping or by doping at a uniformconcentration. Regions of variable refractive index may be formedthrough non-uniform spatial distributions of dopants.

A schematic cross-sectional depiction of a first of many coated fibersin accordance with the present disclosure is shown in FIG. 1. Fiber 11includes core 12, cladding 13, primary coating 16, and secondary coating17. Cladding 13 includes inner cladding region 14 and outer claddingregion 15.

A representative refractive index profile for the glass portion (coreand cladding regions) of an optical fiber is presented in FIG. 2. FIG. 2shows a relative refractive index profile for a fiber having a core (1)with outer radius r₁ and relative refractive index Δ₁, an inner claddingregion (2) extending from radial position r₁ to radial position r₂ andhaving relative refractive index Δ₂, and an outer cladding region (3)extending from radial position r₂ to radial position r₃ and havingrelative refractive index Δ₃. Core region (1) has the highest relativerefractive index in the profile. Core region (1) may include a lowerindex region at or near the centerline (known in the art as a“centerline dip”) (not shown). In the embodiment shown in FIG. 2, outercladding region 3 is directly adjacent inner cladding region 2, which isdirectly adjacent core 1.

The relative ordering of relative refractive indices Δ₁, Δ₂, and Δ₃satisfy the condition

Δ₁>Δ₃×Δ₂

where Δ₁ is greater than zero and each of Δ₂ and Δ₃ may be equal tozero, less than zero or greater than zero. When an offset region ispresent in the relative refractive index profile, the relativerefractive indices Δ₁, Δ₄, and Δ₂ satisfy the condition

Δ₁>Δ₄>Δ₂

where Δ₁ is greater than zero and each of Δ₄ and Δ₂ may be equal tozero, less than zero, or greater than zero.

In one aspect, the present fibers provide low attenuation and higheffective area (A_(eff)). Low attenuation is achieved in part throughspatial uniformity of dopant to minimize Rayleigh scattering. HighA_(eff) fibers are achieved in part by controlling the relativerefractive index Δ₁ of the core to the ranges described hereinbelow. Asis known in the art, however, fibers having high A_(eff) are moresensitive to bending and exhibit higher attenuation due to bendinglosses than fibers having low A_(eff). To counteract the increase inbending losses anticipated for the present high A_(eff) fibers, therelative refractive index profile of the present fibers has beendesigned to include an inner cladding region having a large moat volume.

It is further desirable to achieve fibers having high A_(eff), lowattenuation, low bending losses and a low stress-optic effect. Thestress-optic effect refers to stresses that arise in the core during thefiber draw process. The stresses act to reduce relative refractive indexof the core and thus to reduce the differential in relative refractiveindex between the core and cladding. Since an adequate differential inrelative refractive index between the core and cladding is needed forefficient confinement and waveguiding, it is desirable to minimize thestress-optic effect.

The stress-optic effect in the core is determined by two primaryfactors: (1) core stresses arising from differences in the viscosity ofthe core and cladding and (2) core stresses arising from differences inthe coefficients of thermal expansion of the core and cladding. Forfibers with claddings having multiple regions, the cladding directlyadjacent to the core is of greatest relevance. For purposes of thepresent description, the stress-optic effect is described for a fiberhaving a core and a cladding with an inner cladding region and an outercladding region. The discussion will accordingly focus on stress-opticeffects as they relate to a core with a directly adjacent inner claddingregion.

The fiber draw process entails heating a fiber preform having a core andcladding (including inner cladding region and outer cladding region) toa temperature at or near the softening point and drawing the fiber. Atthe draw temperature, the viscosity of the core and inner claddingregion differ due to differences in composition. The difference inviscosity creates a stress at the interface of the core and innercladding regions. The stress is transmitted to the core and is retainedas the fiber cools and solidifies during draw. Core stress arising fromthe viscosity mismatch between core and inner cladding region is onecontribution to the stress-optic effect.

A second contribution to the stress-optic effect originates from thedifference in coefficient of thermal expansion of the core and innercladding regions. Differences in coefficient of thermal expansion leadto differences in the change in volume of the core relative to the innercladding region as the fiber cools from the softening point andsolidifies during draw. Materials with a high coefficient of thermalexpansion experience greater contraction in volume upon cooling thanmaterials with a low coefficient of thermal expansion. At the softeningpoint, the core and inner cladding regions are sufficiently viscous topermit structural relaxation and dissipation of stresses and differencesin coefficient of thermal expansion are unimportant. As the fiber cools,however, to form a solidified fiber during draw, differentialcontraction caused by differences in the coefficients of thermalexpansion of the core and inner cladding regions produces an interfacialstress that causes stress to develop in the core.

Depending on compositions, the two primary stress contributions to thestress-optic may have greater or lesser effect on the state of stress incore. In some instances, the two effects can act cumulatively toincrease the level of stress in the core, while in other instances, thetwo effects can partially offset or counteract each other to provide anet core stress that is less than the stress of each effectindependently.

The compositions of the core and inner cladding regions of the presentfibers have been selected to minimize the level of stress in the core.In particular, dopants for the core and inner cladding regions, dopingconcentrations, and relative refractive index profiles have beenselected to minimize the stress-optic effect while simultaneouslyproviding high effective area and/or low bending losses.

In one embodiment, the core is silica glass doped with Cl (chlorine). Inanother embodiment, the inner cladding region is undoped silica glass orsilica glass doped with F (fluorine). In still another embodiment, theouter cladding region is undoped silica glass or silica glass doped withF or Cl. In a further embodiment, the core is silica glass doped with Cland free of Ge and K.

In one embodiment, the core is silica glass doped with Cl, the innercladding region is silica glass doped with F and the outer claddingregion is undoped silica glass. In another embodiment, the core issilica glass doped with Cl, the inner cladding region is silica glassdoped with F and the outer cladding region is silica glass doped withCl. In still another embodiment, the core is silica glass doped with Cl,the inner cladding region is silica glass doped with F and the outercladding region is silica glass doped with a lower concentration of Fthan the inner cladding region. In yet another embodiment, the core issilica glass doped with Cl, the inner cladding region is undoped silicaglass and the outer cladding region is silica glass doped with a lowerconcentration of Cl than the core. In each of the foregoing embodiments,the core may be free of Ge and K.

Use of Cl as an updopant in silica glass for the core is preferably touse of Ge. Ge is difficult to incorporate uniformly in silica glass andis susceptible to concentration fluctuations that act to increaseattenuation through Rayleigh scattering. Cl can be incorporated atuniform concentrations as a dopant in silica glass and leads to reducedRayleigh scattering and decreased attenuation.

Cl-doped silica glass is preferable to undoped silica glass as a fibercore material because Cl-doped silica glass has a lower viscosity thanundoped silica glass and more closely matches the viscosity of dopedsilica glass cladding materials to reduce the contribution to corestress arising from a differential in viscosity between the core andinner cladding region. Effective waveguiding by the fiber requires anadequate differential in relative refractive index of the core and innercladding region. When silica glass is the base glass of the fiber, thedifferential in relative refractive index of the core and inner claddingregion can be achieved by updoping the core and downdoping the cladding.Of the possible downdopants for silica glass, F is preferred because itsconcentration distribution can be controlled and relatively uniformdoping with F can be achieved. As in the case of Cl doping, doping ofsilica glass with F leads to a reduction in viscosity at the softeningpoint.

Regarding the influence of thermal expansion on core stress, Cl-dopedsilica glass has a higher coefficient of thermal expansion than undopedsilica glass and F-doped silica glass has a coefficient of thermalexpansion similar to that of undoped silica glass and less than that ofCl-doped silica glass.

Doping concentrations of Cl and F can be varied to control the viscosityand coefficient of thermal expansion of the core and inner claddingregion, respectively. In one embodiment of the present doping scheme,the core and inner cladding region are doped with sufficientconcentrations of Cl and F, respectively, to insure an adequate indexdifferential between the core and cladding, while minimizing thestress-optic effect by controlling the effects of thermal expansion andviscosity in such as a way that the stresses produced by the two effectsat least partially offset each other to lower the net stress in thecore. On the one hand, as the Cl doping concentration of the coreincreases, the coefficient of thermal expansion of the core increases.As a result, for a given doping concentration of F in the inner claddingregion, the contribution of thermal expansion to core stress increases.On the other hand, as the F doping concentration of the inner claddingregion increases for a given Cl doping concentration in the core, theviscosity of the inner cladding region decreases relative to the coreand the contribution of viscosity mismatch to core stress increases.Since the viscosity of the inner cladding region decreases relative tothe viscosity of the core with increased F doping, however, the stresseffect resulting from the viscosity mismatch counteracts the stresseffect associated with the larger coefficient of thermal expansion inthe core relative to the inner cladding region. By balancing the twocontributions to core stress, low core stresses can be achieved and thestress-optic effect can be minimized. The doping concentrations of Cl inthe core and F in the inner cladding region disclosed herein have beenselected to promote balancing of the two contributions. The dopingconcentration of Cl in the core and the core radius r₁ have also beencontrolled to insure large effective area (A_(eff)).

The doping concentration of Cl in the core may be at least 0.5 wt %, orat least 0.8 wt % or at least 1.0 wt %, or at least 1.25 wt %, or atleast 1.5 wt %, or at least 1.75 wt %, or at least 2.0 wt %, or at least2.25 wt %, or in the range from 0.5 wt %-3.0 wt %, or in the range from1.0 wt %-2.75 wt %, or in the range from 1.5 wt %-2.5 wt %. The dopingconcentration of F in the inner cladding region may be in the range from0.10 wt %-0.50 wt %, or in the range from 0.15 wt %-0.45 wt %, or in therange from 0.20 wt %-0.40 wt %. The doping concentration of Cl in thecore may be in the range from 0.5 wt %-3.0 wt % and the dopingconcentration of F in the inner cladding region may be in the range from0.10 wt %-0.50 wt %. The doping concentration of Cl in the core may bein the range from 1.0 wt %-2.75 wt % and the doping concentration of Fin the inner cladding region may be in the range from 0.15 wt %-0.45 wt%. The doping concentration of Cl in the core may be in the range from1.5 wt %-2.5 wt % and the doping concentration of F in the innercladding region may be in the range from 0.20 wt %-0.40 wt %. In each ofthe foregoing embodiments, the core may be free of Ge and K.

In other embodiments, the core is silica glass doped with Cl and theinner cladding region is undoped silica glass. In these embodiments, thestress-optic effect is minimized through counteracting contributions ofmismatches in thermal expansion and viscosity to the core stress. TheCl-doped core has a higher coefficient of thermal expansion and a lowerviscosity than the undoped silica inner cladding region. As a result,the core stresses arising from thermal expansion offset or partiallyoffset core stresses arising from the viscosity differential to providea reduced net core stress and lowering of the stress-optic effect.

The doping concentration of Cl in the silica glass core may be in therange from 2.0 wt %-3.0 wt % and the inner cladding region may beundoped silica glass, or the doping concentration of Cl in the silicaglass core may be in the range from 2.2 wt %-2.8 wt % and the innercladding region may be undoped silica glass, or the doping concentrationof Cl in the silica glass core may be in the range from 2.3 wt %-2.7 wt% and the inner cladding region may be undoped silica glass. In each ofthe foregoing embodiments, the core may be free of Ge and K.

The outer cladding region may be undoped silica glass, updoped silicaglass, or downdoped silica glass. In one embodiment, the outer claddingregion is silica glass doped with Cl and the Cl concentration is in therange from 0.10 wt %-0.60 wt %, or in the range from 0.20 wt %-0.50 wt%. In another embodiment, the outer cladding region is silica glassdoped with F and the F concentration is in the range from 0.05 wt %-0.30wt %, or in the range from 0.10 wt %-0.25 wt %.

The compositions of the core, inner cladding region, and outer claddingregion can be controlled to provide a relative refractive index profileconsistent with fibers having high effective area (A_(eff)) and lowbending losses.

The effective area is controlled in part by the relative refractiveindex Δ₁ of the core, the radius r₁ of the core, and/or the relativerefractive index difference Δ₁−Δ₂ between the core and inner claddingregion. Bending losses are controlled in part by the relative refractiveindex Δ₂ of the inner cladding region, the radius r₂ of the innercladding region, the radial difference r₂−r₁, the trench volume of theinner cladding region, the relative refractive index difference Δ₁−Δ₂between the core and inner cladding region and/or the relativerefractive index difference Δ₃−Δ₂ between the inner cladding region andthe outer cladding region.

The relative refractive index Δ₁ of the core may be in the range from0.08% to 0.30%, or in the range from 0.10% to 0.25%, or in the rangefrom 0.12% to 0.20%, or in the range from 0.14% to 0.18%. The radius r₁of the core may be in the range from 5.0 μm to 9.0 μm, or in the rangefrom 6.0 μm to 10.0 μm, or in the range from 6.0 μm to 9.0 μm, or in therange from 6.0 μm to 8.0 μm, or in the range from 6.5 μm to 7.5 μm, orin the range from 7.0 μm to 10.0 μm.

The relative refractive index Δ₂ of the inner cladding region may be inthe range from 0% to −0.25%, or in the range from −0.05% to −0.20%, orin the range from −0.10% to −0.20%. The radius r₂ of the inner claddingregion may be in the range from 15 μm to 40 μm, or in the range from 15μm to 38 μm, or in the range from 20 μm to 38 μm, or in the range from20 μm to 35 μm, or in the range from 20 μm to 30 μm, or in the rangefrom 22 μm to 38 μm, or in the range from 22 μm to 35 μm, or in therange from 24 μm to 38 μm, or in the range from 24 μm to 35 μm.

The trench volume of the inner cladding region may be greater than 20%Δμm², or greater than 30% Δμm², or greater than 40% Δμm², or greaterthan 60% Δμm², or greater than 80% Δμm², or greater than 100% Δμm², orin the range from 20% Δμm² to 200% Δμm², or in the range from 30% Δμm²to 170% Δμm², in the range from 40% Δμm² to 140% Δμm².

The relative refractive index Δ₃ of the outer cladding region may be inthe range from −0.20% to 0.10%, or in the range from −0.15% to 0.10%, orin the range from −0.10% to 0.05%, or in the range from −0.05% to 0.05%.The radius r₃ of the outer cladding region may be at least 55 μm, or atleast 60 μm, or in the range from 55 μm to 70 μm, or in the range from60 μm to 65 μm, or about 62.5 μm.

The relative refractive index difference Δ₁−Δ₂ may be at least 0.15%, orat least 0.20%, or at least 0.25%, or at least 0.30%. The relativerefractive index difference Δ₃−Δ₂ may be at least 0.05%, or at least0.06%, or at least 0.08%, or at least 0.10% or at least 0.12%, or atleast 0.15%, or at least 0.20%.

The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.05%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.06%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.08%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.10%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.12%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.15%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.20%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.25%.

The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.05%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.06%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.08%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.10%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.12%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.15% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.20%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.20%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.20% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.25%.

The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.05%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.06%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.08%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.10%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.12%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.15%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.20%.The relative refractive index difference Δ₁−Δ₂ may be at least 0.25% andthe relative refractive index difference Δ₃−Δ₂ may be at least 0.25%.

Optical fibers with the doping schemes and relative refractive indexprofiles described herein feature high mode field diameters, largeeffective areas, low attenuation, and low bending losses.

The mode field diameter of the fiber at a wavelength of 1550 nm may beat least 10.0 μm, or at least 11.0 μm, or at least 11.5 μm, or at least12.0 μm, at least 12.5 μm, or at least 13.0 μm, or at least 13.5 μm, orat least 14.0 μm, or in the range from 10.0 μm to 15.0 μm, or in therange from 11.0 μm to 14.0 μm.

The effective area of the present fibers at a wavelength of 1550 nm maybe at least 100 μm², or at least 110 μm², or at least 120 μm², or atleast 130 μm², or at least 140 μm², or at least 150 μm², or in the rangefrom 100 μm² to 180 μm², or in the range from 110 μm² to 165 μm², or inthe range from 120 μm² to 155 μm².

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 20 mm may be lessthan 4.0 dB/turn, or less than 3.5 dB/turn, or less than 3.0 dB/turn, orless than 2.5 dB/turn, or less than 2.0 dB/turn, or less than 1.5dB/turn, or less than 1.0 dB/turn, or between 0.2 dB/turn and 4.0dB/turn, or between 0.3 dB/turn and 3.5 dB/turn, or between 0.4 dB/turnand 3.0 dB/turn, or between 0.5 dB/turn and 2.5 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 30 mm may be lessthan 1.5 dB/turn, or less than 1.0 dB/turn, or less than 0.8 dB/turn, orless than 0.6 dB/turn, or less than 0.4 dB/turn, or less than 0.3dB/turn, or less than 0.2 dB/turn, or between 0.05 dB/turn and 1.5dB/turn, or between 0.1 dB/turn and 1.5 dB/turn, or between 0.15 dB/turnand 1.0 dB/turn, or between 0.2 dB/turn and 0.8 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 40 mm may be lessthan 0.6 dB/turn, or less than 0.4 dB/turn, or less than 0.2 dB/turn, orless than 0.1 dB/turn, or less than 0.05 dB/turn, or less than 0.025dB/turn, or less than 0.01 dB/turn, or between 0.005 dB/turn and 0.6dB/turn, or between 0.005 dB/turn and 0.5 dB/turn, or between 0.01dB/turn and 0.5 dB/turn, or between 0.01 dB/turn and 0.4 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 50 mm may be lessthan 0.25 dB/turn (25 dB/100 turns), or less than 0.10 dB/turn (10dB/100 turns), or less than 0.05 dB/turn (5.0 dB/100 turns), or lessthan 0.020 dB/turn (2.0 dB/100 turns), or less than 0.010 dB/turn (1.0dB/100 turns), or less than 0.005 dB/turn (0.5 dB/100 turns), or lessthan 0.002 dB/turn (0.2 dB/100 turns), or less than 0.001 dB/turn (0.1dB/100 turns), or between 0.0005 dB/turn and 0.25 dB/turn, or between0.001 dB/turn and 0.20 dB/turn, or between 0.001 dB/turn and 0.15dB/turn, or between 0.002 dB/turn and 0.15 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by themandrel wrap test using a mandrel having a diameter of 60 mm may be lessthan 0.15 dB/turn, or less than 0.10 dB/turn, or less than 0.05 dB/turn,or less than 0.02 dB/turn, or less than 0.01 dB/turn, or less than 0.005dB/turn, or less than 0.002 dB/turn, or between 0.001 dB/turn and 0.15dB/turn, or between 0.001 dB/turn and 0.10 dB/turn, or between 0.002dB/turn and 0.15 dB/turn, or between 0.002 dB/turn and 0.10 dB/turn.

The bending loss of the present fibers at 1550 nm as determined by thewire mesh covered drum microbending test is less than 2.0 dB/km, or lessthan 1.0 dB/km, or less than 0.5 dB/km, or between 0.25 dB/turn and 2.0dB/turn, or between 0.25 dB/turn and 1.0 dB/turn, or between 0.5 dB/turnand 2.0 dB/turn, or between 0.5 dB/turn and 1.5 dB/turn.

The attenuation of the present fibers at 1550 nm may be less than 0.20dB/km, or less than 0.19 dB/km, or less than 0.18 dB/km, or less than0.17 dB/km, or less than 0.16 dB/km.

The cable cutoff wavelength of the present fibers may be less than 1600nm, or less than 1550 nm, or less than 1500 nm, or less than 1450 nm, orless than 1400 nm.

The dispersion of the present fibers at 1550 nm may be no more than 26ps/nm/km, or no more than 24 ps/nm/km, or no more than 22 ps/nm/km, orno more than 22 ps/nm/km.

The core and cladding of the present fibers may be produced in asingle-step operation or multi-step operation by methods which are wellknown in the art. Suitable methods include: the flame combustionmethods, flame oxidation methods, flame hydrolysis methods, OVD (outsidevapor deposition), IVD (inside vapor deposition), VAD (vapor axialdeposition), double crucible method, rod-in-tube procedures,cane-in-soot method, and doped deposited silica processes. A variety ofCVD processes are known and are suitable for producing the core, innercladding region, and outer cladding region used in the optical fibers ofthe present invention. They include external CVD processes, axial vapordeposition processes, modified CVD (MCVD), inside vapor deposition, andplasma-enhanced CVD (PECVD).

Suitable precursors for silica include SiCl₄ and organosiliconcompounds. Organosilicon compounds are silicon compounds that includecarbon. Organosilicon compounds may also include oxygen and/or hydrogen.Examples of organosilicon compounds include OMCTS(octamethylcyclotetrasiloxane), silicon alkoxides (Si(OR)₄),organosilanes (SiR₄), and Si(OR)_(x)R_(4-x), where R is acarbon-containing organic group or hydrogen and where R may be the sameor different at each occurrence, subject to the proviso that at leastone R is a carbon-containing organic group. Suitable precursors forchlorine doping include Cl₂, SiCl₄, Si₂Cl₆, Si₂OCl₆, SiCl₃H, and CCl₄.Suitable precursors for fluorine doping include F₂, CF₄, and SiF₄.

Optical fibers disclosed herein can be made by forming a preform,consolidating the preform, and drawing a fiber. By way of example andnot intended to be limiting, formation of a silica (or doped silica)soot preform according to the OVD method is illustrated in FIGS. 3 and4. In FIG. 3, soot preform 20 is formed by depositing silica-containingsoot 22 onto the outer surface of a rotating and translating bait rod24. Bait rod 24 is preferably tapered. The soot 22 is formed byproviding a glass/soot precursor 28 in gaseous form to the flame 30 of aburner 26 to oxidize it. Fuel 32, such as methane (CH₄), and combustionsupporting gas 34, such as oxygen, are provided to the burner 26 andignited to form the flame 30. Mass flow controllers, labeled V, meterthe appropriate amounts of glass/soot precursor 28, fuel 32 andcombustion supporting gas 34, all preferably in gaseous form, to theburner 26. The glass/soot precursor 28 is a glass former compound and isoxidized in the flame 30 to form the generally cylindrically-shaped sootregion 23, which may correspond to the core of an optical fiber preform.

After forming of the soot core preform, as illustrated in FIG. 4, thesoot core preform 20 including the cylindrical soot region 23 may bedoped (e.g. with chlorine) and sintered or consolidated in furnace 29 toform a sintered or consolidated soot core preform. Prior to sintering orconsolidation, the bait rod 24 illustrated in FIG. 3 is removed to forma hollow, cylindrical soot core preform. During the chlorine doping andsintering or consolidation process, the soot core preform 20 issuspended, for example, inside a pure quartz muffle tube 27 of thefurnace 29 by a holding mechanism 21. Prior to the sintering orconsolidation step the preform 20 is exposed to a chlorine-containingatmosphere. For example, a suitable chlorine doping atmosphere mayinclude about 0 percent to 70 percent helium and 30 percent to 100percent chlorine gas, in some embodiments 50 percent to 100 percentchlorine gas, at a temperature of between about 950° C. and 1500° C. anda suitable doping time ranges from about 0.5 and 10 hours.

The fibers disclosed herein utilize high chlorine doping concentrations.High chlorine doping levels can be achieved by controlling a number ofvariables. For example, higher temperatures may be used to vaporizeliquid SiCl₄ (a chlorine doping precursor), resulting in increased SiCl₄concentration in the vapor phase. The vaporizer temperature in someembodiments is higher than 40° C., in some other embodiments is higherthan 45° C., in some other embodiments is higher than 50° C. and in yetother embodiments is higher than 57° C. As a result, increased SiCl₄concentration may be employed in the consolidation furnace. In someembodiments, the fraction of the gas through the vaporizer/bubbler tothe total flow to the furnace is higher than 30%, in other embodiments,the fraction of the gas through the vaporizer/bubbler to the total flowto the furnace is higher than 50% and in yet other embodiments, thefraction of the gas through the vaporizer/bubbler to the total flow tothe furnace is higher than 80%. The remainder of the gas may be helium.In certain other embodiments, the fraction of the gas through thevaporizer/bubbler to the total flow to the furnace is 100%

In some embodiments, doping with SiCl₄ or other Cl-doping precursoroccurs during the sintering process, i.e. the soot preform is beingdoped prior to and/or up to the point where the soot preform goes toclosed pore state and becomes a fully sintered preform, in presence ofSiCl₄ or other Cl-doping precursor at temperatures higher than 1300° C.,in other embodiments at temperatures higher than 1375° C. In someembodiments the chlorine doping occurs during the sintering process attemperatures higher than 1400° C.

Use of higher soot surface area preforms for doping with SiCl₄ or otherCl-doping precursor is another strategy for increasing the Cl dopingconcentration in the soot preform. In some embodiments, the surface areaof the soot preform is larger than 10 m²/g; in other embodiments, thesurface area of the soot preform is larger than 20 m²/g; in yet otherembodiments, the surface area of the soot preform is larger than 25m²/g; and in still other embodiments, the surface area of the sootpreform is larger than 50 m²/g. In certain other embodiments, thesurface area of the soot preform is larger than 90 m²/g. The surfacearea of the preform can be measured using BET surface areacharacterization techniques.

The amount of chlorine doping using SiCl₄ or other Cl-doping precursorcan also be increased by treating the silica soot preform with multiplecycles of successive exposure of SiCl₄ (or other Cl-doping precursor)and H₂O/O₂ prior to full consolidation of the preform. Without wishingto be bound by theory, it is believed that the treatment of silica sootsurface with SiCl₄ results in doping of chlorine at Si—OH sites on thesilica soot surface through reaction of SiCl₄ and OH to release HCl andform a Si—O—SiCl₃ group. In addition, SiCl₄ can react with Si—O—Silinkages on the surface of the silica soot preform to produce a Si—Clgroup and a Si—O—SiCl₃ group. Each Cl atom in the attached —SiCl₃ groupcan be converted to OH group by treating with water (and/or oxygen),which then in turn become reactive sites for attaching additional —SiCl₃groups upon subsequent treatment with SiCl₄. By exploiting the procedurewhere the preform is exposed to repeated cycles of successive SiCl₄ andH₂O (and/or O₂) treatments, it is possible to create a cascading(branched or fractal-like) surface structure that incorporates highamounts of chlorine. This results in significantly higher chlorinedoping levels in the consolidated glass compared to doped chlorinelevels reported in prior art. Similar considerations apply to otherCl-doping precursors.

Other methods that can be used to increase the soot surface area of thepreform include: 1) low density laydown, 2) pressed high surface areasoot, and 3) impregnating the soot with a sol-gel silica (e.g., TEOS,pre or post hydrolyzed) or nano-particle silica such as Ludox® colloidalsilica.

After the chlorine doping step, the core soot preform may be sintered.The sintering temperatures employed in the present invention preferablycan range from 1100° C. to 1600° C., more preferably between about 1400°C. and 1550° C., and most preferably between about 1480° C. and 1550° C.One particularly preferred sintering temperature is approximately 1490°C. After sintering, the core preform may be drawn to a smaller diameterand cut into lengths to form consolidated chlorine-doped glass corecanes.

Sintering may occur in the presence of SiCl₄ or other Cl-dopingprecursor. The partial pressure of SiCl₄ or other Cl-doping precursorduring Cl-doping and/or sintering may be greater than 0.5 atm, orgreater than 1.0 atm, or greater than 2.0 atm, or greater than 5.0 atm.

The sintered Cl-doped soot may be used as a glass core or glass corecane in optical fiber manufacturing. Additional soot to form the innercladding region may then be deposited onto the glass core or glass corecane using the same method as explained above with respect to the coresoot deposition process. The inner cladding soot can then be doped withfluorine using a dopant gas having fluorine or other optical fiberdopants therein. For example, SiF₄ and/or CF₄ gas may be employed. Suchdopant gases may be employed using conventional doping temperatures, forexample between about 950° C. and 1250° C. for 0.25 to 4 hours. Thesintering temperatures employed in the present invention preferably canrange from 1100° C. to 1600° C., more preferably between about 1400° C.and 1550° C., and most preferably between about 1480° C. and 1550° C.One particularly preferred sintering temperature is approximately 1490°C.

The fibers disclosed herein may be drawn from optical fiber preformsmade using conventional manufacturing techniques and using known fiberdraw methods and apparatus, for example as is disclosed in U.S. Pat.Nos. 7,565,820, 5,410,567, 7,832,675, 6,027,062, the specifications ofwhich are hereby incorporated by reference. In particular, optical fiberis pulled from a root portion of the optical fiber preform by a tractor.After leaving a draw furnace, the bare optical fiber encounters adiameter monitor (D) which provides a signal that is used in a feedbackcontrol loop to regulate speed of the tractor to maintain a constantfiber diameter. The bare optical fiber then passes through a fibertension measurement device (T) that measures the tension of the opticalfiber caused by pulling the fiber from the preform. This tension canincrease depending on the speed of the fiber draw, the temperature andviscosity of the root of the preform, etc. One example of a fibertension measurement device is disclosed in EP 0479120 A2 which is herebyincorporated herein by reference.

EXAMPLES

Exemplary fibers in accordance with the present description are nowdescribed and modeled to illustrate one or more advantageous featuresdisclosed herein.

The exemplary fibers have the relative refractive index profiles shownin FIGS. 5-18. The exemplary fibers included a core region, an innercladding region, and an outer cladding region. The radii and relativerefractive indices of the different regions of the exemplary fibers areshown in Tables 1a, 1b, and 1c. The radius of the outer cladding regionextended to a radius r₃ of 62.5 μm, but the outer cladding region wastruncated in the depictions shown in FIGS. 5-18. Tables 1a, 1b, and 1calso include the trench volume of the inner cladding region of eachexemplary fiber. Units of each parameter and the corresponding figureare listed. Regions with Δ>0 and Δ<0 were obtained by inclusion of Cl asan updopant in silica glass and F as a downdopant in silica glass,respectively. Values of relative refractive index were essentiallylinear with doping concentration and can be estimated from therelationships: 1 wt % F˜−0.32Δ% and 1 wt % Cl˜0.10Δ%. Regions with Δ=0correspond to undoped silica glass. Examples shown in Tables 1a, 1b, and1c had a core alpha of 20. The notation “Ex.” signifies “Example” andprovides a distinguishing reference to each of the Exemplary Fibers1-14. Exemplary Fibers 1-12 lacked an offset region between the core andinner cladding region. Exemplary Fibers 13 and 14 included an offsetregion between the core and inner cladding region.

TABLE 1a Parameter Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 FIG. 5 6 7 8 9 Δ₁ (%)0.2 0.16 0.155 0.2 0.25 r₁ (μm) 6.2 6.7 7.4 7.3 7.4 Δ₂ (%) −0.12 −0.12−0.1 −0.05 0 r₂ (μm) 22 25 22 25 25 V_(Trench) (% Δμm²) 107 139 85 57 62Δ₃ (%) 0 0 0 0.02 0.055 r₃ (μm) 62.5 62.5 62.5 62.5 62.5

TABLE 1b Parameter Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 FIG. 10 11 12 13 14 Δ₁(%) 0.11 0.15 0.15 0.15 0.114 r₁ (μm) 8 6.2 6.2 6.7 6.51 Δ₂ (%) −0.115−0.17 −0.17 −0.13 −0.15 r₂ (μm) 30 29 20 25 23 V_(Trench) (% Δμm²) 92273 39.8 151 34.1 Δ₃ (%) −0.06 0 −0.115 0 −0.115 r₃ (μm) 62.5 62.5 62.562.5 62.5

TABLE 1c Parameter Ex. 11 Ex. 12 Ex. 13 Ex. 14 FIG. 15 16 17 18 Δ₁ (%)0.15 0.13 0.16 0.15 r₁ (μm) 7.4 7.05 6.1 6.1 Δ₄ (%) −0.085 −0.09 r₄ (μm)10 10 Δ₂ (%) −0.1 −0.1 −0.155 −0.17 r₂ (μm) 23 29 24.2 24.2 V_(Trench)(% Δμm²) 95 47.5 34.0 43.7 Δ₃ (%) 0 −0.07 −0.085 −0.09 r₃ (μm) 62.5 62.562.5 62.5

The optical properties of Exemplary Fibers 1-14 were modeled and theresults are presented in Tables 2a, 2b, and 2c.

TABLE 2a Property Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 MFD at 1550 nm (μm)11.785 12.662 13.593 13.571 13.65 A_(Eff) at 1550 nm (μm²) 112.338 130151.3 150.14 152.34 Dispersion at 1550 nm (ps/nm/km) 20.947 21.17321.504 21.3 21.27 Dispersion Slope at 1550 nm 0.0609 0.061 0.061 0.0610.061 (ps/nm²/km) Cable cutoff wavelength (nm) 1400 1400 1440 1430 145020 mm bend loss at 1550 nm (dB/km) 0.9332 1.3974 2.1054 2.0855 2.0497 30mm bend loss at 1550 nm (dB/km) 0.2382 0.2889 1.0351 0.5117 0.3109 40 mmbend loss at 1550 nm (dB/km) 0.0968 0.1337 0.5089 0.1287 0.0603 50 mmbend loss at 1550 nm (dB/km) 0.0394 0.0619 0.2502 0.0324 0.0117 60 mmbend loss at 1550 nm (dB/km) 0.0160 0.0287 0.1230 0.0081 0.0023Attenuation at 1550 nm (dB/km) <0.170 <0.170 <0.170 <0.170 <0.170

TABLE 2b Property Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 MFD at 1550 nm (μm)14.61 11.79 11.79 12.66 12.72 A_(Eff) at 1550 nm (μm²) 175.44 112.43112.43 130.1 130.5 Dispersion at 1550 nm (ps/nm/km) 21.67 21.05 21.0521.2 20.9 Dispersion Slope at 1550 nm 0.061 0.0605 0.0602 0.061 0.061(ps/nm²/km) Cable cutoff wavelength (nm) 1450 1420 1420 1425 1425 20 mmbend loss at 1550 nm (dB/km) 2.8839 0.0952 1.2747 0.4225 2.8570 30 mmbend loss at 1550 nm (dB/km) 0.5375 0.0131 0.0391 0.2228 0.3956 40 mmbend loss at 1550 nm (dB/km) 0.1002 0.0062 0.0034 0.1175 0.0401 50 mmbend loss at 1550 nm (dB/km) 0.0187 0.0029 0.0003 0.0619 0.0041 60 mmbend loss at 1550 nm (dB/km) 0.0035 0.0014 0.0000 0.0327 0.0004Attenuation at 1550 nm (dB/km) <0.165 <0.170 <0.170 <0.170 <0.165

TABLE 2c Property Ex. 11 Ex. 12 Ex. 13 Ex. 14 MFD at 1550 nm (μm) 13.6613.69 12.49 12.51 A_(Eff) at 1550 nm (μm²) 152.5 152.6 123.7 124.3Dispersion at 1550 nm (ps/nm/km) 21.47 21.04 20.6 20.73 Dispersion Slopeat 1550 nm 0.061 0.061 0.062 0.063 (ps/nm²/km) Cable cutoff wavelength(nm) 1430 1430 1486 1480 20 mm bend loss at 1550 nm (dB/km) 3.66655.1873 0.10 0.06 30 mm bend loss at 1550 nm (dB/km) 0.8350 0.7162 0.00690.0062 40 mm bend loss at 1550 nm (dB/km) 0.4105 0.0989 50 mm bend lossat 1550 nm (dB/km) 0.2018 0.0137 60 mm bend loss at 1550 nm (dB/km)0.0992 0.0019 0.00002 0.0002 Attenuation at 1550 nm (dB/km) <0.170<0.165

In Tables 2a, 2b and 2c, MFD refers to mode field diameter, A_(eff)refers to effective area, the cable cutoff wavelength refers to the LP11mode, and bend loss is modeled for the mandrel wrap test using mandrelswith the specified diameters. The optical fiber designs illustrated inTables 1a, 1b, 1c, 2a, 2b, and 2c had effective area at 1550 nm ofgreater than 100 μm², cable cutoff wavelength of less than 1550 nm,dispersion at 1550 nm of less than 22 ps/nm/km, bend loss for a 20 mmmandrel diameter at 1550 nm of less than 3.5 dB/turn, bend loss for a 60mm mandrel diameter at 1550 nm of less than 2 dB/100 turns and anattenuation of less than 0.17 dB/km. These fibers had a wire meshcovered drum microbending loss of less than 2 dB/km at a wavelength of1550 nm. These fibers had wire mesh covered drum microbending loss ofless than 1 dB/km at a wavelength of 1550 nm. The trench volume,V_(Trench), defined as (Δ₂−Δ₃)*(R₂ ²−R₁ ²) in the inventive examples ofTables 1a, 1b, 1c, 2a, 2b, and 2c was greater than 50% Δμm². In someembodiments, the trench volume, V_(Trench), is greater than 100% Δμm².In still other embodiments, the trench volume, V_(Trench), is greaterthan 150% Δμm². In certain embodiments, the optical fibers include aprimary coating having a Young's modulus of less than 1 MPa and asecondary coating having a Young's modulus of greater than 1200 MPa. Insome embodiments, the optical fibers include a primary coating having aYoung's modulus of less than 0.5 MPa and a secondary coating having aYoung's modulus of greater than 1500 MPa.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the illustrated embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments that incorporate the spirit and substance of the illustratedembodiments may occur to persons skilled in the art, the descriptionshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of making an optical fiber comprising:forming soot from an organosilicon compound; and doping said soot withCl, and sintering said doped soot, said sintered doped soot having a Clconcentration of at least 0.8 wt %.
 2. The method of claim 1, whereinsaid doping includes heating said soot to a temperature higher than1300° C.
 3. The method of claim 1, wherein said soot has a surface arealarger than 50 m²/g.
 4. The method of claim 1, wherein said dopingincludes exposing said soot to a Cl-doping precursor, said Cl-dopingprecursor comprising a compound selected from the group consisting ofSiCl₄, Si₂Cl₆, Si₂OCl₆, SiCl₃H, Cl₂, and CCl₄.
 5. The method of claim 4,wherein said Cl-doping precursor is SiCl₄.
 6. The method of claim 5,wherein said doping comprises maintaining a partial pressure of saidCl-doping precursor of greater than 0.5 atm.
 7. The method of claim 5,wherein said doping comprises maintaining a partial pressure of saidCl-doping precursor of greater than 1.0 atm.
 8. The method of claim 5,wherein said doping comprises maintaining a partial pressure of saidCl-doping precursor of greater than 2.0 atm.
 9. The method of claim 5,wherein said doping comprises maintaining a partial pressure of saidCl-doping precursor of greater than 5.0 atm.
 10. The method of claim 5,wherein said Cl-doping precursor is a vapor, said vapor formed byheating a liquid comprising said Cl-doping precursor.
 11. The method ofclaim 10, wherein said liquid is heated at a temperature higher than 40°C.
 12. The method of claim 1, wherein said soot is formed as a core sootpreform.
 13. The method of claim 1, wherein said sintered doped soot hasa Cl concentration of at least 1.5 wt %.
 14. The method of claim 1,wherein said sintered doped soot has a Cl concentration of at least 2.0wt %.
 15. The method of claim 1, wherein said sintered doped soot has aCl concentration of at least 2.25 wt %.
 16. The method of claim 1,wherein said sintered doped soot lacks Ge and K.
 17. The method of claim1, further comprising depositing second soot on said sintered dopedsoot.
 18. The method of claim 17, further comprising doping said secondsoot with F.
 19. The method of claim 17, further comprisingconsolidating said second soot.
 20. The method of claim 1, furthercomprising drawing said sintered doped soot.
 21. The method of claim 1,wherein said organosilicon compound is octamethyltetrasiloxane.